1. Field of the Invention
The present invention relates to a single-sideband transmitter including a single-sideband generator and an amplifier connected to each other via a phase-modulation (PM) path and an amplitude-modulation (AM) path, and to a method for operating this transmitter.
2. Description of the Prior Art
In the case of a conventional amplitude-modulated medium wave or short wave transmitter, the radiated frequency spectrum of the carrier frequency when amplitude-modulated with a low-frequency signal contains, apart from this carrier frequency, two sidebands of which one is located only a little above and the other only a little below the carrier frequency. Each of these sidebands has the band width of the original low-frequency band and each taken singly completely contains the information of the low-frequency signal. Since the total transmitted power is distributed over both sidebands and the carrier frequency, the power used for transmitting the information is disproportionately high and the efficiency relatively low with reference to the information transmitted. In addition, the transmission of both sidebands also makes the channel width required at least twice as great as with single-sideband operation even though no additional information is transmitted in the additional band.
To reduce the power required in the transmitter and to reduce the channel width, single-sideband transmitters have long been used, especially for small mobile radio stations. However, the saving in energy obtained by single-sideband operation gains special significance in high-power broadcast transmitters in which the use of energy is a significant factor in the calculation of operating costs. Early attempts therefore were made to operate broadcast transmitters with single-sideband modulation. In such attempts, already existing amplitude-modulation transmitters were frequently modified for single-sideband transmission in the following manner: the amplitude-modulation modulator, in most cases a class B push-pull amplifier which has in its output a modulation transformer, was cut off. Single-sideband modulation was carried out at a low power level at the radio-frequency source of the transmitter, that is, for example, at the carrier-frequency oscillator or synthesizer. The transmitter then worked as a linear power amplifier for the single-sideband signal, the amplifier stages being changed over to class B mode from the class C mode customary for amplitude modulation.
It is true that this type of modification considerably reduced the channel width of the transmitter. However, the achievable peak of the envelope power was only about 50% of the rated power of the transmitter. If in addition a residual carrier was also radiated, the single-sideband power was possibly even less than that achievable in the amplitude-modulation mode of operation. For the rest, the overall efficiency of the transmitter deteriorated considerably as a result of the transition to class B operation in the output stage so that the usefulness of such a change-over was at best marginal.
Another possibility of single-sideband operation consists in generating the single-sideband at a high power level in the final stage of the transmitter by means of a suitable combination of amplitude and frequency modulation. Such a method is described, for example, in the article by O. Villard, Electronics, Vol. 21, November 1948, pp 86 et seq. The single-sideband system illustrated there consists of a combined amplitude and phase modulation in which a constant phase shift of 90.degree. is maintained between the two low-frequency modulation signals which are otherwise identical. As a result of this phase shift, the two upper or lower sidebands are largely suppressed in the resulting frequency spectrum. In practice, however, it has been found to be extremely difficult to construct suitable phase shifters which guarantee that the required 90.degree. phase shift occurs over the wide frequency range of a broadcast transmitter.
Another type of single-sideband transmitter is known from the article by L. Kahn, Proc. I.R.E., July 1952, pp.803 et seq. There too the starting point is a combined amplitude and phase modulation which leads to an amplified single-sideband signal in the final stage of the transmitter. But it differs from the aforementioned single-sideband method in that here, instead of using two similar low-frequency modulation signals which are phase shifted by 90.degree., the single-sideband signal is generated in a single-sideband generator and consists of one amplitude-modulated component and one phase-modulated component. The single sideband signal is split into these components at low power. The phase-modulated component is separately amplified and used as a phase-modulated carrier. The amplitude-modulated component is also separately amplified in a conventional push-pull class B modulator and used in known manner to modulate the phase-modulated carrier as an envelope curve. The combination of the two components again produces the original single-sideband signal in amplified form.
In the single-sideband transmitter described, problems occur in the general case where the low-frequency signal is an audio signal with a mean amplitude value which changes with time. In this case, the amplitude-modulated component of the single-sideband signal contains a direct-voltage component which changes with time and which cannot be transferred via the modulation transformer of the amplitude modulator. For this reason, an additional circuit for floating-carrier modulation must be provided which contains, for example, a series of d.c.-voltage amplifiers and modulates via a valve grid the final stage of the transmitter as a function of the d.c.-voltage component mentioned. The carrier amplitude can be modulated in this manner only from its nominal value to zero. The upper half wave, on the other hand, cannot be implemented in this way. Because of the considerable additional expenditure which is involved in achieving full floating-carrier modulation, this type of transmitter is essentially used for telegraphy with frequency-shift keying because no need exists there for floating-carrier modulation, because of the constant signal amplitudes, and the requirements on the linearity of the transmitter are relatively low.